A flexure endurant composition of elastomer reinforced with a continuous phase of microporous, expanded polytetrafluoroethylene (ePTFE).
Silicone elastomers can be fabricated into many forms for use, illustratively, in the medical, electrical, and chemical industries. Articles such as peristaltic pump tubes, pump diaphragms, bellows, baby bottle nipples, wire and cable sheaths, gaskets, and O-rings, for example, are commonly made from silicone elastomers. Many of these articles, moreover, are used in applications that require repeated flexing. For example, peristaltic pumps are used to transport liquids and pastes through an elastomeric tube in which the tube is squeezed between a set of rotating rollers and a fixed pump housing. Silicone elastomers are frequently used for peristaltic pump tubing. Upon repeated flexure, however, the silicone rubber tubing develops cracks in the side wall and ruptures catastrophically. The problem is exacerbated when pumping fluids at elevated pressures and temperatures, leading to even shorter pump tubing life. Clearly, a more durable substance is needed for these purposes.
Silicones are a class of inherently flexible polymers with organosilicon-oxygen repeating units which undergo bond rotation with little resistance. As a result, silicones possess excellent low temperature properties; however, their weak intermolecular and intramolecular polymer interactions result in poor tear strength and toughness. As a result, silicone elastomers are often reinforced with either particulate inorganic fillers or soluble silicone resin fillers. Inorganic fillers, such as fumed silica, for example, are known to increase the tensile strength of dimethyl silicones by a factor of ten. Even the best silicone elastomers, however, are still limited to approximately 1,300 psi tensile strength (ASTM D-412) and 250 ppi tear strength (ASTM D-624 die B). Natural rubber, on the other hand, has significantly higher tensile and tear properties; however, it lacks many of the useful silicone elastomer attributes of low temperature flexibility, low dielectric loss, ozone resistance, low extractables, and radiation resistance. Thus, the need for an improved class of reinforced silicone elastomers that combine the strength and toughness of natural rubber with the useful attributes of silicone rubber continues to be unsatisfied.
Polytetrafluoroethylene (PTFE) is a polymer with excellent chemical inertness coupled with high strength. In U.S. Pat. Nos. 3,953,566; 3,962,153; 4,096,227; and 4,187,390, Gore teaches the lubrication of PTFE powder and subsequent expansion of the PTFE into a microstructure characterized by nodes interconnected by fibrils. In these patents, Gore teaches the use of non-reactive fluids such as kerosene, naphtha, or mineral spirits as the lubricating fluid to aid in the extrusion of PTFE fine powder. The PTFE is extruded into a tape and dried to remove the non-reactive lubricant. Finally, the extrudate is expanded to produce a material that has both high porosity and high strength.
Expanded PTFE (ePTFE) has also been prepared using reactive lubricants, as seen in Mitchell (U.S. Pat. No. 4,764,560), and Tu (EP 256,748; U.S. Pat. No. 5,071,609). Reactive lubricants consist of uncured silicone and optionally a solvent such as kerosene, naphtha, or mineral spirits. The PTFE fine powder is lubricated, extruded, and expanded. During the expansion process, the silicone cures in situ to form an interpenetrating polymer network (IPN) of PTFE and silicone elastomer. Such expanded structures have residual porosity, high strength, and moderate resilience. Mitchell (U.S. Pat. No. 4,764,560; U.S. Pat. No. 4,891,407; WO 87/02996) and Dillon (U.S. Pat. No. 4,832,009; WO 9117205), for example, teach the use of heat curable dimethylsilicone to produce a porous microstructure of interpenetrating matrices in cured form with moisture vapor transmission properties for use as bandages for severe burn victims. The amount of curable silicone suggested in Mitchell""s ""560 and ""407 patents can range from as little as 1 part by weight per 100 parts of PTFE to as much 150 parts of silicone per 100 parts of PTFE. Using Mitchell, however, it is not feasible to expand paste extruded tape having more than 20 weight percent silicone into a microstructure of interpenetrating matrices in cured form due to the lack of interconnection between nodes and fibrils which results in poor extrudate green strength. Thus, the compositions described by Mitchell possessed little elasticity due to their relatively high volume fraction of PTFE when compared to the present invention.
Tu (U.S. Pat. No. 4,816,339) also describes the use of reactive and unreactive lubricants for the preparation of radially asymmetric vascular grafts having an elastomer content ranging from 5 to 120 weight percent ratio of elastomer relative to PTFE. Tu teaches the use of fluoroelastomers, silicone elastomers, and others. A typical process used for producing a multi-layer PTFE/elastomer implant included blending the PTFE fine powder with the solvated elastomer, preforming a multilayered billet, extruding out of a die, curing the elastomer, expanding the composite, and forming an optional elastomeric polymer coating layer via a dip or spray coating operation. Other tubular prostheses have been developed by Mano (U.S. Pat. No. 4,304,010) which comprise a porous tubing of PTFE having a microstructure composed of fibrils and nodes connected to one another by the fibrils, the fibrils being radially distributed, and a porous coating of an elastomer bound to the outside surface of said PTFE tubing. The prosthesis can be vacuum impregnated with elastomer solution to provide a coating thickness of between 20 and 500 microns. The prosthesis has improved suture tear resistance when compared to previous art.
Tomoda (U.S. Pat. No. 4,133,927) teaches the lamination of ePTFE to an elastomer substrate wherein the porous sheet of ePTFE forms a layer having a thickness of about 0.05 mm or more on the surface of the elastomer substrate. The composite is formed by superimposing the porous film or sheet on a vulcanizable rubber elastomer substrate and subjecting the material to heat and pressure sufficient to affect vulcanization of the rubber and adhesion between the porous PTFE and the elastomer substrate. In the case of fluorine-containing rubber, the resulting composite exhibits excellent chemical resistance. Tomoda does not teach the use of multiple layers of ePTFE to form a composite that is capable of transferring stress on a molecular level throughout the bulk.
For many years, silicone elastomers have been modified with PTFE powder to increase their lubricity, thermal stability, and tear strength. Safford (U.S. Pat. No. 2,710,290) teaches the use of a minor portion of solid PTFE dispersed throughout the silicone to form randomly distributed fibrils. He shows that the PTFE particles were elongated in situ within the silicone matrix by means of shear deformation action. As a result, the tear strength, as measured by ASTM D-624 (die B), was increased from 65 ppi to 230 ppi. Konkle teaches in U.S. Pat. No. 2,927,908 that PTFE can be used to increase tensile and tear strength in heat curable fluorinated organopolysiloxane elastomers. These composites were also characterized as fuel and oil resistant. These examples of PTFE particles dispersed into silicone rubber are limited to less than 25 weight percent due to the difficulty in processing of the rubber and deterioration of the physical properties of the vulcanizate. Unlike the present invention, PTFE powder filled elastomers lack the continuous layer of ePTFE whose microstructure can be characterized by nodes interconnected by fibrils, and thus have inferior flexure resistance.
It is an object of the present invention to provide a composite elastomer with superior flexure endurance wherein the ratio of elastomer to ePTFE ranges from 1:1 to 50:1 on a volume basis.
Another object of the present invention is to provide a chemical resistant fluoroelastomer for applications that require flexure in the presence of aggressive chemicals.
Still another object of the present invention is to provide a reinforced elastomer useful for pump components, diaphragms, gaskets, seals, o-rings, belts, tubes, and bellows.
The present invention relates to flex endurant elastomer compositions based on elastomers reinforced with a continuous phase of microporous or expanded polytetrafluoroethylene (ePTFE). More particularly, the invention relates to a mixture of ingredients comprising (1) a liquid elastomer convertible to a cured, solid elastic state and (2) a minor portion of ePTFE having a continuous microstructure characterized by nodes interconnected by fibrils.
There are also provided methods for fabricating these flex endurant composites of an elastomer and ePTFE. The processes involve coating ePTFE material with liquid elastomer, wrapping the impregnated material around a mandrel, and, optionally, applying heat and/or pressure to vulcanize the elastomer.